But a new website for the PAC attacking Republican gubernatorial hopeful Charlie Baker is set to launch Tuesday, uncloaking the groups backing it. The site says the PAC is “supported by the Massachusetts Teachers Association, 1199 SEIU United Healthcare Workers East, 32BJ SEIU, and the Democratic Governors Association.” Those groups include some of the state’s most politically active unions, along with the national organization dedicated to electing Democratic governors.

Their involvement marks a new chapter in a gubernatorial race widely expected to be nasty and expensive, including millions of dollars in spending from outside groups aligned with both Democrats and Republicans but functioning independently of candidates.

The website, shown to the Globe before its launch, offers familiar attacks on Baker, who faced a fusillade of negative television ads from groups aligned with Democrats during his unsuccessful 2010 bid to unseat Governor Deval Patrick. The site knocks him for his tenure as chief executive of Harvard Pilgrim Health Care and his involvement with the Big Dig as state budget chief during the mid-1990s.

The site is also set to include videos, one of which paints Baker as a profiteering health insurance executive who looked out for his financial interests at the expense of consumers.

In response to a description of the website, Baker spokesman Tim Buckley said in a statement that the eventual Democratic nominee will have to defend the state’s dysfunctional health insurance exchange website and billions of dollars in new spending.

“The sad truth is that the Democratic candidates have nothing positive to talk about, while Charlie Baker has the experience and skills to make Massachusetts great again,” Buckley said.

Democratic consultant Stephen Crawford, a spokesman for the PAC, said it spent more than $100,000 assembling the website and producing the videos. He said the point of the website, bakerfacts.com, is to provide an easily accessible central repository of information on Baker’s record for voters. Crawford added that all the information includes citations explaining its sourcing.

Asked whether the site was meant as a repository of tested attacks for other Democratic organizations to use against Baker, Crawford did not say.

“None of this information is new to the public domain, but it’s presented here in an easily understandable format,” he replied.

He declined to reveal whether the PAC planned to air television advertisements, saying only that “we’re going to use every modern campaign tool at our disposal.”

Asked how much the
Mass Independent Expenditure Political Action Committee planned to spend this year, Crawford would only say, “yet to be determined.”

Baker and Shrewsbury businessman Mark R. Fisher are vying for the GOP nomination for governor. Five Democrats are in the race, including Attorney General Martha Coakley, who was endorsed last week by 1199SEIU. Also running for governor are a Libertarian and three independent candidates.